The same thing went on in my truck when the 40 gal. tank was installed, I would be driving after my fill up and the DTE would increase 100 miles or more. I just watch my fuel gage and did not relay on the DTE and filled the truck when fuel le vel wasat ¼ of a tank to go. I had my tank installed last October 2019 and I made one long trip with the truck 1500 miles round trip plus whatever local miles I put while I was there for a few weeks. At this time, I believe the DTE is correct. I do not have any more functionating with the DTE moving up and down. A few tanks of fuel is all it takes. I have never ran the tank down to show only 100 miles to go to empty.